Due to the developments in CMOS technology, sigma delta modulators are widely used nowadays. Sigma delta modulator can be employed for audio signal applications, in receiver devices, such as mobile phones, and the like.
FIG. 1 shows a first embodiment of a sigma delta modulator according to prior art. As can be seen from this Figure, an input signal x[n] can be received via an input terminal 6 by an integrator device 2. The integrator device comprises an adding unit and a delay unit 3. The output signal v[n] of the integrator device 2 is fed via connection 8 to a quantizer device 4, which comprises an output terminal 10. The integrator device 2 receives as the input signal the difference value between the input signal x[n] and the output signal of the quantizer device 4.
In addition, the order of a sigma delta modulator is defined by the number of provided integrators. For instance, the sigma delta modulator according to FIG. 1 is a first order sigma delta modulator. To increase the order of the sigma delta modulator, several approaches are known. For instance, as shown in FIG. 2, the integrators 2 can be cascaded with one quantizer device 4 in a single structure. Additionally, comparing units 12 can be arranged.
Another possibility of a higher order sigma delta modulator is shown in FIG. 3. This Figure shows a multistage noise shaping (MASH) structure. A plurality of integrator devices each connected to a quantization device is arranged in cascaded form. The arrangement of an integrator device and a quantizer device 4 is also called a stage of the sigma delta modulator. The quantization error can be expressed by addition of white Gaussian noise (AWGN) sources (not shown). The output of each stage is fed to an error cancellation network 24. The error cancellation network 24 comprises delay units 18.1 and 18.2 and delay units 20. The number of delay cells in a delay unit 18.1 or 18.2 for a certain stage is N-n, wherein N is the order of the sigma delta modulator and n is the number of the respective stage. The error cancellation network 24 may be configured to reduce the quantization error.
The previously described embodiments of a sigma delta modulator according to prior art comprise the basic issue that their output spectra have spurious frequencies. In the present context spurious frequencies may be regarded as frequencies with an amplitude that is significantly higher than the noise spectrum of the sigma delta modulator. Such frequencies may cause problems in several applications. By way of example, in case the sigma delta modulator is used in a PLL, wherein the PLL is a local oscillator within a tuner system, spurious frequencies may yield to spurious reception. According to a further example, if the PLL is employed as a clock source for an analogue digital converter (ADC) or digital analogue converter (DAC), spurious frequencies may cause spurious tones or whistles.
For preventing spurious frequencies in the output spectrum of a sigma delta modulator according to prior art, in particular a sigma delta modulator having the order three or higher and a constant input value or signal, three conditions must be fulfilled. The first condition is that the bit width is large enough for good approximation of an irrational starting condition and long repetition period. The second condition is that the superposition of the output signals of the sigma delta modulator stages is performed with proper phase relation in the error cancellation network. The third condition is that an irrational starting condition is given.
The first and the second conditions can be fulfilled, for instance, by the implementation of a sigma delta modulator according to the embodiment shown in FIG. 3. The irrational starting condition may result in that the value cannot be calculated from a fraction of integers and is a decimal which does not comprise a periodic fractional part. The successful operation of sigma-delta modulator relies on the assumption that the successive input samples are uncorrelated. In case of constant input values, an irrational starting condition may cause an irrational operation mode resulting in a spurious free output spectrum. More particularly, an irrational starting condition or irrational operation mode may be very effective in randomizing the binary quantization error, and thus, spurious frequencies can be prevented.
However, during runtime of the sigma delta modulator, its operation mode can be changed from an irrational operation mode to a rational operation mode. In particular, due to disturbances, such as electrostatic discharge, power dip, cosmic or radioactive radiation, the phase relation between at least two stages can be destroyed or the system can go into another operation mode generating an output spectrum with spurious frequencies.
Therefore, it is one object of the present application to provide a method for preventing negative effects caused by disturbances during the runtime of the sigma delta modulator. Another object is to prevent spurious frequencies in an output spectrum of a sigma delta modulator. A further object of the present application is to prevent destroying of phase relation.